The invention relates to a percent body fat measuring apparatus for measuring percent body fat on the basis of bio-impedance, and more particularly, to a percent body fat measuring apparatus which measures bio-impedance by causing a signal analogous to a half-wave waveform to flow through a living body.
As described in, e.g., the Japanese Patent No. 2835656, the Unexamined Japanese Patent Application Publication Nos. Heill-104104, Heill-113872, and 2001-212098, according to related-art techniques for measuring bio-impedance, a sinusoidal waveform of 50 KHz or the like is used as a measurement signal to be caused to flow through a living body (hereinafter called a “measurement signal”). A voltage stemming from flow of the measurement signal through the living body is extracted through use of an electrode to be brought into contact with the living body. Next, after being amplified by a differential amplifier, the voltage is converted into a d.c. voltage through use of a rectifier circuit. A d.c. level of the voltage is read through use of an analog-to-digital converter (this technique is taken as a first “related-art”).
Under the assumption that a circuit shown in FIG. 15 is used as the rectifier circuit, when a voltage of operating power is set to, e.g., four volts, a range of variation in the level of a rectified output assumes a value of about 1.5 volts, because of influence of a forward voltage of a diode which performs rectification. Specifically, this results in occurrence of a deficiency in the rectified output. With a view toward eliminating such a problem, a circuit shown in FIG. 13 is put forth. Specifically, a scan be seen from an input and an output of the circuit shown in the drawing, when an input varies from zero volts to two volts, an output varies from four volts to two volts. Accordingly, when the voltage of the operating power is set to four volts, the range of variation in the level of the rectified output can be set to two volts. Specifically, the circuit prevents narrowing of the range of variation in the level of the rectified output, which would otherwise be caused under the influence of the forward voltage of the diode which performs rectifying operation (this technique is taken as a second related-art technique).
Further, when bio-impedance is measured, a measurement signal, such as an a.c. signal, is caused to flow through a living body, and a voltage stemming from flow of the measurement signal through the living body is detected. However, the thus-detected voltage sustains the influence of an error of a value of the electric current flowing through the living body or the influence of an error arising in a detection circuit for detecting a voltage. A related-art technique for solving such an error is put forward in Japanese Patent No. 2835656. According to this technique, a plurality of groups of reference resistors are inserted in current paths which permit flow of the measurement signal into the living body. Voltage drop values corresponding to the respective reference resistors, which differ in resistance value from each other, and a voltage drop value corresponding to the living body are measured through use of a single circuit under the same measurement environment. From the values of the respective reference resistors and the voltage drop values corresponding to the respective reference resistors, a correlation between a voltage drop value and impedance is determined. The thus-determined correlation is applied to a voltage drop value of the living body, to thereby compute bio-impedance. The error of the value of the electric current flowing through the living body and the error arising in the detection circuit for detecting a voltage are eliminated. Consequently, the thus-measured bio-impedance assumes a highly accurate value (this technique is taken as a first related-art technique).
A configuration shown in FIG. 21 is also put forward, by the present applicant. More specifically, signal generator 503 for producing a measurement signal whose waveform is analogous to a half-wave waveform is provided, and a generated measurement signal is applied to a first electrode 501. A second electrode 502 is grounded by way of a detection resistor R514. The level of a half-wave of a half-wave signal developing between terminals of the detection resistor R514 is detected by use of level detector 504. The level detector 504 is configured from only discrete elements which are inexpensive (this technique is taken as a second related-art technique).
As is touched on in the description of the second related-art technique, the voltage of the operating power for an apparatus which displays percent body fat on the basis of measured bio-impedance (i.e., a percent body fat measuring apparatus) is set to a low voltage such as four volts. In order to facilitate transport of the apparatus, a battery is used as a power source. According to the first related-art technique, an amplitude of the measurement signal is limited to a value as low as four volts. In contrast, a signal detected from the living body is rectified after being amplified. Specifically, only a half-wave component of the sinusoidal wave becomes effective for measuring bio-impedance. Put another way, even when the operating power is set to four volts and when the amplitude of the measurement signal is also set to four volts, only a component corresponding to two volts is utilized as an effective component. As described in connection with the second related-art technique, a rectifier circuit formed from a complicated circuit configuration is required. Even when a rectifier circuit having such a complicated circuit configuration is used, the range of variation in the rectified output is limited to two volts. Therefore, when the level of a signal developing in the living body as a result of flow of the measurement signal through the living body is detected, difficulty is encountered in improving a detection accuracy.
Further, according to the second related-art technique, when a temperature rises, a base current of a p-n-p transistor Q501 also increases. Hence, an output level of the collector of the transistor rises in association with the temperature rise. A forward voltage of a diode D506 decreases in association with the temperature rise. Even when the level of the collector remains constant, a rectified output 651 is increased in response to the temperature rise. Consequently, as a whole, the temperature change induces occurrence of synergistic action between an increase in the base current of the p-n-p transistor Q501 and a decrease in the forward voltage of the diode D506, thereby deteriorating a temperature characteristic. According to the second related-art technique having such a characteristic, the level of the signal developing between the terminals of the detection resistor R514 directly indicates bio-impedance Z to be measured. Hence, the method for eliminating an error according to the first related-art technique cannot be applied to the second related-art technique. Therefore, demand arises for another method for enhancing accuracy of detection of the bio-impedance Z.